1. Field of Invention
The field of the currently claimed embodiments of this invention relates to optical coherence tomography systems, and more particularly to Fourier domain optical coherence tomography systems that have real-time artifact and saturation correction.
2. Discussion of Related Art
Optical coherence tomography (OCT) has been viewed as an “optical analogy” of ultrasound sonogram (US) imaging since its invention in early 1990's (D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. A. Puliafito, and J. G. Fujimoto, “Optical coherence tomography,” Science, vol. 254, pp. 1178-1181, 1991). Compared to the conventional image-guided interventions (IGI) using modalities such as magnetic resonance imaging (MRI), X-ray computed tomography (CT) and ultrasound (US) (T. Peters and K. Cleary, Image-Guided Interventions: Technology and Applications, Springer, 2008), OCT has much higher spatial resolution and therefore possesses great potential for applications in a wide range of microsurgeries, such as vitreo-retinal surgery, neurological surgery and otolaryngologic surgery.
As early as the late 1990's, interventional OCT for surgical guidance using time domain OCT (TD-OCT) at a slow imaging speed of hundreds of A-scans/s has been demonstrated (S. A. Boppart, B. E. Bouma, C. Pitris, G. J. Tearney, J. F. Southern, M. E. Brezinski, J. G. Fujimoto, “Intraoperative assessment of microsurgery with three-dimensional optical coherence tomography,” Radiology, vol. 208, pp. 81-86, 1998). Due to the technological breakthroughs in Fourier domain OCT (FD-OCT) during the last decade, ultrahigh-speed OCT is now available at >100,000 A-scan/s. For example, see the following:                B. Potsaid, I. Gorczynska, V. J. Srinivasan, Y. Chen, J. Jiang, A. Cable, and J. G. Fujimoto, “Ultrahigh speed Spectral/Fourier domain OCT ophthalmic imaging at 70,000 to 312,500 axial scans per second,” Opt. Express, vol. 16, pp. 15149-15169, 2008.        R. Huber, D. C. Adler, and J. G. Fujimoto, “Buffered Fourier domain mode locking: unidirectional swept laser sources for optical coherence tomography imaging at 370,000 lines/s,” Opt. Lett., vol. 31, pp. 2975-2977, 2006.        W-Y. Oh, B. J. Vakoc, M. Shishkov, G. J. Tearney, and B. E. Bouma, “>400 kHz repetition rate wavelength-swept laser and application to high-speed optical frequency domain imaging,” Opt. Lett., vol. 35, pp. 2919-2921, 2010.        B. Potsaid, B. Baumann, D. Huang, S. Barry, A. E. Cable, J. S. Schuman, J. S. Duker, and J. G. Fujimoto, “Ultrahigh speed 1050 nm swept source/Fourier domain OCT retinal and anterior segment imaging at 100,000 to 400,000 axial scans per second,” Opt. Express, vol. 18, pp. 20029-20048, 2010.        W. Wieser, B. R. Biedermann, T. Klein, C. M. Eigenwillig, and R. Huber, “Multi-Megahertz OCT: High quality 3D imaging at 20 million A-scans and 4.5 GVoxels per second,” Opt. Express, vol. 18, pp. 14685-14704, 2010.        T. Klein, W. Wieser, C. M. Eigenwillig, B. R. Biedermann, and R. Huber, “Megahertz OCT for ultrawide-field retinal imaging with a 1050 nm Fourier domain mode-locked laser,” Opt. Express, vol. 19, pp. 3044-3062, 2011.        
For a spectrometer-based SD-OCT, an ultrahigh speed CMOS line scan camera based system has achieved up to 312,500 line/s in 2008 (Potsaid et al.); while for a swept laser type OCT, >20,000,000 line/s rate was achieved by multi-channel FD-OCT using a Fourier Domain Mode Locking (FDML) laser in 2010 (Wieser et al.).
Fourier-domain optical coherence tomography (FD-OCT) is a high-speed high-resolution three-dimensional imaging modality widely used in biomedical imaging. For OCT to find applications in the interventional imaging area, real-time image processing and display are required. The presence of the fixed-pattern-noise artifact that forms strong erroneous horizontal lines laying over the image is among the most common types of noise in FD-OCT systems (R. A. Leitgeb and M. Wojtkowski, “Complex and coherent noise free Fourier domain optical coherence tomography,” in Optical Coherence Tomography, Technology and Applications, Wolfgang Drexler, and James G. Fujimoto, eds, 177-207 (Springer, 2008)). The fixed-pattern-noise artifact can be removed if the reference spectrum of that imaging frame is known. Therefore, in the case of high-resolution OCT imaging of a fixed site, simple subtraction of the reference spectrum from the OCT signal spectra works very effectively. However, the source spectrum shape varies over time; the OCT signal level and the spectra vary frame by frame; the optical phase and polarization can also vary frame to frame. Therefore, an effective signal processing method that can remove the DC levels due to these changes in real-time from the image data and thus remove the fixed pattern noise is of great importance in improving the quality of OCT images (Sucbei Moon, Sang-won Lee and Zhongping Chen, ‘Reference spectrum extraction and fixed-pattern noise removal in optical coherence tomography’, Opt. Express 18, 24395-24404 (2010); Bernd Hofer, Boris Pova{hacek over (z)}ay, Boris Hermann, Sara M. Rey, Vedran Kajić, Alexandre Tumlinson, Kate Powell, Gerald Matz, and Wolfgang Drexler, “Artefact reduction for cell migration visualization using spectral domain optical coherence tomography,” J. Biophotonics 4, 355-367 (2011)). However, periodically measuring and verifying the reference spectrum for OCT video imaging is highly inconvenient and impractical.
Saturation artifacts occur when light reflected back from a highly specular surface, generating signals that are over the dynamic range of the data acquisition system (Hiram G. Bezerra, Marco A. Costa, Giulio Guagliumi, Andrew M. Rollins and Daniel I. Simon, “Intracoronary optical coherence tomography: a comprehensive review,” JACC: Cardiovascular interventions 2, 1035-1046 (2009)). It is not uncommon to see optical coherence tomography images that are corrupted by saturations artifacts, for example in cornea imaging (Sanjay Asrani, Marinko Sarunic, Cecilia Santiago, Joseph Izatt, “Detailed visualization of the anterior segment using Fourier-Domain optical coherence tomography,” Arch Ophthalmol 126, 765-771 (2008); Francesco LaRocca, Stephanie J. Chiu, Ryan P. MeNabb, Anthony, N. Kuo, Joseph A. Izatt, and Sina Farsiu, “Robust automatic segmentation of corneal layer boundaries in SDOCT images using graph theory and dynamic programming,” Biomedical Optics Express 2(6), 1524-1538 (2011)), intracoronary imaging (Bezerra et al.), and finger pad imaging (Michael A. Choma, Kevin Hsu and Joseph A. Izatt, “Swept source optical coherence tomography using an all-fiber 1300-nm ring laser source,” Journal of Biomedical Optics 10(4), 044009 (2005)). Real-time removal of the saturation artifacts can increase the diagnostic and interventional accuracy. There thus remains a need for improved Fourier domain optical coherence tomography systems that have real-time artifact and saturation correction.